Anion Exchange Membranes with Improved Chemical Stability

Chulsung Bae

Department of Chemistry & Chemical Biology

Rensselaer Polytechnic Institute

CFES Symposium (02/26//2015, 25 min) Contact: baec@rpi.edu

Collaborators: Yu Seung Kim (Los Alamos National Laboratory) Chang Y. Ryu (Rensselaer Polytechnic Institute) Michael Hickner (Pennsylvania State University)

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Clean Energy Ion-conducting (H+, OH-) polymer electrolytes for fuel cells

Functional Polymer Synthesis Polymerization of functional monomers Controlled polymer modifications

Overview of Bae Group Research

Clean Environment • Water purification

• CO2 capture

• Polymer-supported

recyclable catalysts

Functional Organic/Polymeric

Materials forEnvironment &

Energy

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Fuel Cells: PEMFC and AEMFC

• Highest power density • PEM: insufficient conductivity at low RH high cost of Nafion • Catalyst: expensive Pt

• Lower power density than PEMFC • AEM: insufficient conductivity poor stability against OH-

• Catalyst: non-noble metals, Ni, Co

Reaction at Anode: H2 2 H+ + 2 e- H2 + 2 OH- 2 H2O + 2 e-

Reaction at Cathode: ½ O2 + 2 H+ + 2 e- H2O ½ O2 + H2O + 2 e- 2 OH-

Overall Reaction: H2 + ½ O2 electricity + H2O H2 + ½ O2 electricity + H2O

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Synthetic Methods Toward Functional Polymers

Specific Applications: Electronic, Biomedical, Energy-generation, etc

Light-weight Flexible Shape Mechanical Stability

Reduced reactivity Low molecular weight Distribution of FG

Side reactions (degradation, XL) Molecular weight control

Low Cost

High Value

Direct Borylation of Aromatic C–H Bonds

• Iridium-catalyzed aromatic C-H bond activation/borylation • Boron substitutes only aromatic C–H bonds selectively • Mixture of meta and para-borylated products

Miyaura & Hartwig, JACS 2002, 124, 390 Smith, JACS 2000, 122, 12868

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Synthetic Applications of Borylated Arene: Intermediate for Functionalized Arenes

Smith, JACS 2003, 125, 7792 Miyaura, Tetrahedron 2008, 64, 4967 Hartwig, Org. Lett. 2007, 9, 761

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Ir-Catalyzed C–H Borylation of Polysulfone: Degree of Functionalization & Molecular Weight Property

Entry B2pin2 /PSU

Bpin (mol %)

Effic. (%)

1 0.2 23 57 2 0.4 64 80 3 0.6 101 84 4 0.8 126 79 5 1.0 147 74 6 1.2 175 73 7 1.4 196 70 8 1.6 206 65

Jo et al, JACS 2009, 131, 1656 Chang et al, Macromolecules 2013, 46, 1754

A: [IrCl(COD)]2 B: [Ir(OMe)(COD)]2

Mn = 25.2 kg/mol $0.7/g (Sigma-Aldrich)

THF

B2pin2/PSU 1.6 1.2 0.4 0

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Sulfonated Polysulfone for High Temp Low RH PEMs

135%-SO3H 160%-SO3H 200%-SO3H

T. S. Jo et al, JACS 2009, 131, 1656

Chang et al., Polym. Chem. 2013, 4, 272-282 Ying Chang 8

Water Absorption Properties & Proton Conductivity of Sulfonated Polysulfones

Michael Hickner

@ 100 oC

IEC 0.89 1.94 2.57 2.29

Ying Chang 9

Anion Exchange Membrane (AEM) for Fuel Cells

Metal catalysts: Fe, Co, Ni, Pt Fast cathode reaction rates Fuel flexibility / Low fuel crossover

Advantages:

AEM

Common polymer materials

Common cations

Low ion conductivity (<100 mS/cm) Poor chemical & mechanical stabilities in high pH and >80 oC

Issues of OH– conducting polymers

Inexpensive Commercially available Easy to modify

Easy to prepare Reasonably good stability

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Chemical Degradation of AEM by OH- Attack

Degradation at Polymer Backbone » Chain cleavage at C–O–C

OH- is an aggressive base/Nu » SN2 substitution

» Ylide intermediate formation

» E2 (β-Hofmann) elimination

Degradation at Cation Group

Kim, J. Membr. Sci. 2012, 423-424, 438 Ramani, PNAS 2013, 110, 2490 Hickner, ACS Macro Lett. 2013, 2, 49 11

In alkaline condition, degradation occurs at both cation group and polymer backbone simultaneously!

Pivovar, J. Phy. Chem. C. 2010, 114, 11977

Molecular Engineering Approach to Improve AEM Stability

Angela Mohanty

Increased steric hindrance

Resonance-stabilized Utilize polymer backbones

made of only C–C bond

Hibbs et al.

Macromolecules (2009) 42, 8316

Approach: decouple the stabilities of cation and polymer backbone 1. Investigate cation stability systematically using model QAs

– Sterically hindered cations, resonance-stabilized cations 2. Employ more stable polymer backbones

– No C–O bonds, high molecular weights 3. Incorporate stable QA structures to non-degradable polymer

Cations Polymer Backbone

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NMR Study of QA Model Compounds (1) Ion exchange & isolate QA in OH- form (2) Transfer to NMR tube

& immerse in preheated oil bath for 1 mo.

(3) Record NMR spectrum periodically

0 h 1 d 6 d

11 d 18 d

28 d

D2O

a b c

d a b c d

18-crown-6

Degradation product

Angela Mohanty

A. Mohanty & C. Bae J. Mater. Chem. A. 2014, 2, 17314

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Quantitative Stability Comparison of Small Molecule QAs

Angela Mohanty 14

Summary Ion-conducting aromatic polymers with different cation/anion structures

synthesized by combination of C-H borylation & Suzuki coupling Convenient controls of structure & concentration of ionic groups

QA Model Compounds Stability • Systematic quantitative stability study of QAs • Sterically hindered QAs (-CH2N+R3) are more stable • Long alkyl-tethered QAs are more stable than benzyl-tethered QAs

Anion Exchange Membrane • Polymer chains made of C–C bonds: no backbone degradation • PF-, SEBS-QA (where QA = -CH2N+R3) • Good chemical stability with high OH– conductivity • Excellent fuel cell performance

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Acknowledgment

Current and Past Group Members • Postdoc: Ying Chang, Woo-Hyung Lee Dongwon Shin • Graduate student: Angela Mohanty,

Bhagyashree Date, Sarah Park, Stefan Turan, Jihoon Shin, Se Hye Kim

Collaborators

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• Molecular Dynamics: Seung Soon Jang (Georgia Tech)

• AFM, Mechanical Property, MEAs: Yu Seung Kim (Los Alamos National Laboratory)

• Water Absorption, SAXS: Michael A. Hickner (Penn State)

• SAXS: Joel Morgan, Chang Y. Ryu (RPI)

• Ir and Pd catalysts: Sino Chemicals • B2pin2: Frontier Scientific